Deadlock in Lwt_io due to simultaneous flushes

[vouillon]

The program below never terminates, despite the exit 0. What happens is that exit 0 attempts to flush the channel while Lwt_io.flush oc is still in progress.

let (>>=) = Lwt.bind
let flags =  [Unix.O_WRONLY; Unix.O_CREAT; Unix.O_APPEND; Unix.O_NONBLOCK]
let _ =
Lwt_main.run
  (Lwt_unix.openfile "/tmp/out.txt" flags 0o666 >>= fun fd ->
   let oc = Lwt_io.of_fd ~mode:Lwt_io.output fd in
   ignore (Lwt_io.write oc "BUG" >>= fun () -> Lwt_io.flush oc);
   exit 0)